Dynamoelectric machines, such as electric motors and generators, which convert electrical energy into mechanical power and mechanical energy into electrical power, respectively, comprise a stationary member, also called a stator or field, and a rotating member, also called a rotor or armature. The stator and rotor each have a magnetic structure. An electric motor employs electricity to create a magnetic charge on the magnetic structure of the stator, which in turn causes the rotor to rotate, creating mechanical power. The magnetic structure of both the stator and rotor employ a conductor, usually aluminum, and thin laminations or poles, usually made of steel. The production and wear of stators and rotors can cause a number of weaknesses or faults, including shaft wobble, magnetic centering, worn bearings, pole distortions, electrical shorts in motor windings, leakage between windings and porosity or impurities in the magnetic structures, which can detrimentally affect output and reliability.
Various technologies are known in the art to minimize the negative impact motor failures have on the safety, reliability, and production of motors. While some of these known methods have employed undesirable destructive testing techniques, others use non-destructive techniques to test stators and rotors for defects. One such test measures inductance versus rotor position. According to this test, the rotor is slowly turned while a known electrical signal is applied to the inductive windings of a motor. The known signal is reflected as an electromagnetic field. The manner in which characteristics, such as frequency, amplitude and waveform, of an applied field are reflected represents an electromagnetic signature of a particular motor. This electromagnetic signature is analyzed to detect dissymmetries, which can be used to predict defects in the motor. The electromagnetic signature can also be chronicled against historical motor data to predict future motor failures.
Systems are known in the art that employ portable computers to map, record and analyze the reflected electromagnetic signatures. This use of an electromagnetic signature to determine potential problems before motor failure results in reduced downtime, and is analogous to catscans or magnetic resonance images which are used to determine potential health problems in humans. However, the longer a motor is used, the longer the stator and rotor are magnetized, resulting in the creation of residual magnetic fields on the magnetic structures of the stator and motor. Therefore, the base inductance of a motor increases over its lifecycle. Residual magnetic fields can be substantial in large dynamoelectric machines such as motors larger than 100 horsepower. Current inductance tests are based on data garnered from the observation of small motors, where the residual magnetism of the stator and rotor is not large enough to skew tests.
Current inductance tests and devices are skewed by the lack of residual magnetic field correction, and therefore tend to overidentify older machines, particularly large motors, as problematic. Existing test procedures recognize that an offset problem exists, but rather than incorporating a solution, such procedures initialize the test with the rotor in the same position each time: for example, consistently starting with the rotor key notch in an upright position. This allows consistency when multiple tests are conducted within a short period of time on the same motor. However, this procedure does not allow for comparison tests between similar motors as the residual magnetic field of every motor varies. Further, as the residual magnetic field increases with use, the test history of a particular motor cannot be reliably chronicled.
A need exists for a simple and inexpensive device that can reliably detect the residual magnetic field of a motor, allowing for more accurate determination of stator and rotor anomalies.